The major factors which determine whether a Si.sub.3 N.sub.4 powder can be sintered are not only its purity, the proportion of .alpha.-Si.sub.3 N.sub.4 phase present in the powder and the particle form of the powder but also especially the particle size.
Powders having a BET surface area (nitrogen method) greater than 10 m.sup.2 /g are regarded as suitable for sintering. The average particle size of the agglomerates in these powders should be less than 1 .mu.m.
It is also known that hard agglomerates have an extremely adverse effect on the sintering process. Powders in which the agglomerates are hard but not dense can only be compressed to low green densities. This results in severe, uncontrollable shrinkage in the manufacture of the ceramic component as well as prolonged sintering times and hence the risk of the development of coarse grains. In addition, the coarse structures present in the green product can no longer be eliminated in the course of the sintering process and the strength properties are therefore seriously impaired.
In the known manufacturing processes employed industrially, Si.sub.3 N.sub.4 powders are obtained in an agglomerated form. The average size of the agglomerates, which may be determined, for example, by laser diffraction methods, is generally in the region of 1 to 10 .mu.m. Before sintering is carried out, the agglomerates of these Si.sub.3 N.sub.4 powders must be broken down to an average particle size of less than 1 .mu.m.
Various processes of wet grinding (attrition milling, ball milling, etc) have been recommended for breaking down the agglomerates of Si.sub.3 N.sub.4 powders but these methods are only useful to a limited extent. Although the agglomerates can be broken down, it is not possible to prevent abrasion of the grinding bodies, and consequently the Si.sub.3 N.sub.4 powders become contaminated with the grinding dust. When ceramic grinding bodies (e.g. Al.sub.2 O.sub.3 , ZrO.sub.2) were used, the powders were found to contain large grains in the form of splinters probably consisting of material which had broken away from the grinding balls. Although contamination by a foreign substance could be prevented by using grinding balls consisting of Si.sub.3 N.sub.4, the grinding section of Si.sub.3 N.sub.4 balls is relatively weak so that the grinding times are very prolonged, amounting to several hours, and the costs of the process are therefore high.
Apart from the above mentioned harmful effects on the Si.sub.3 N.sub.4 powders, the re-agglomeration which takes place in the course of drying is regarded as a general disadvantage of wet grinding. Dry agglomerates may seriously impair the capacity of ground powder to undergo sintering.
It has also been proposed (Alpine Aktuell No. 26, Alpine/Augsburg) to disagglomerate Si.sub.3 N.sub.4 by grinding in a counter-jet mill. The Si.sub.3 N.sub.4 powders are to be disagglomerated to such a degree in a fluidized bed counter-jet mill that a residue of 1% is left after the material has been passed through a 10 .mu.m sieve. Fluidized bed counter-jet mills, however, have a moving sifter which is subject to considerable wear due to the high abrasiveness of the silicon nitride powder. When grinding tests are carried out on Si.sub.3 N.sub.4 powder having an initial iron content of less than 100 ppm, the iron content is found to be increased after the test to 500 ppm which amounts to an unacceptable degree of contamination for a good quality Si.sub.3 N.sub.4 powder.
It is further stated in Sprechsaal 118 (6), 525-528 (1985) that Si.sub.3 N.sub.4 may be ground in counter-jet mills with an integrated sifter and the abrasion on the sifter can be prevented. According to the examples given, however, the desired degree of fineness corresponding to an average particle size of less than 1 .mu.m cannot be achieved. Even when less hard materials are ground in these mills, e.g. lead glass, the d.sub.50 value obtained after 6 passages through the mill is still 1.5 .mu.m. It is also known from the literature that it was virtually impossible to alter the dispersivity of commercial Si.sub.3 N.sub.4 powders by air jet grinding (Dissertation G. Wotting, Berlin 1983).
In our copending application Ser. No. 53,957, filed May 22, 1987, we disclose and claim sinterable Si.sub.3 N.sub.4 powders containing sintering additives and a process for their preparation.
It is an object of the present invention to provide a Si.sub.3 N.sub.4 powder which can readily be sintered and which does not have the disadvantage of powders prepared by the known processes.